Devices having a balloon mounted at the distal end of a catheter are useful in a variety of medical procedures. A balloon reservoir may be used to deliver a biologically compatible fluid, such a radiologically opaque fluid for contrast x-rays, to a site within the body. Radial expansion of a balloon may be used to expand or inflate a stent positioned within the body. A balloon may also be used to widen a vessel into which the catheter is inserted by dilating the blocked vessel. For example, in the technique of balloon angioplasty, a catheter is inserted for long distances into blood vessels of extremely reduced diameter and used to release or dilate stenoses therein by balloon inflation. These applications require extremely thin walled high strength relatively inelastic balloons of accurately predictable inflation properties.
Dilatation balloons made from PET (polyethylene terephthalate) are well known and widely used for angioplasty, stent placement, treatments in the gastrointestinal, urethral, or reproductive tracts, and for other medical purposes. Other polymer materials have also been reported to be useful in such applications and some of those polymer materials have also been used commercially, for instance, polyethylene, polyvinyl chloride, Surlyn® polyethylene ionomer copolymer, nylon 12, Pebax® polyamide-polyether-polyester block copolymer, and polyester-polyether block copolymers.
Polymer balloons for medical devices are conventionally made by radially expanding a tubular parison of a polymer material at a temperature above the glass transition temperature (Tg) of the material. (In the present application “the glass transition temperature” when referring to a material which has more than one glass transition refers to the highest glass transition temperature displayed by the material). Sometimes the tubular parison is simply an extruded tube. However, frequently the extruded tube is axially stretched prior to being radially expanded. Axial stretching may be undertaken at ambient or at a temperature above ambient temperature. See U.S. Pat. No. 4,490,421 to Levy.
In U.S. Pat. No. 5,017,325 and U.S. Pat. No. 5,223,205, a process and apparatus are described for making balloons of polyamide polymer. In the process an extruded tubing segment is subjected to an initial stretching step, an initial blowing step, and then to at least one further stretching step and at least one further blowing step.
In WO 99/44649, corresponding to U.S. Pat. No. 6,465,067, both of which are incorporated here by reference, there is described a process for blowing balloons of a material having a high crystallization rate from an extruded tubing segment. The process involves stretching the tubing in a manner which produces a propagating necking of at least a portion of the tubing segment while simultaneously subjecting the tubing to an internal pressure above ambient and then radially expanding the necked portion of the tubing at a temperature above the Tg of the material. It has been observed that in the practice of this process, the necking step reduces both the exterior diameter and the interior diameter of the tubing segment.